Do any of the other boards offer an exam that is roughly in between the standard of grade 8 and DipABRSM? I'm taking Grade 8 viola in the winter, but I know I would never have the time to prepare for DipABRSM (I'm sitting DipABRSM piano in the summer), and I wondered if there was anything else I could work towards before leaving for university in a couple of years' time. What about Trinity Performer's Certificate?

Trinity do a Performer's Certificate which fits in the gap between Grade 8 and diploma exams. Details are on their website.

I know you're really talking about Viola quallies, but if you're an organist, why not try the Royal College of Organists' CertRCO ? It provides a very accessible route into the demands of the subsequent diplomas (ARCO, FRCO)

Thanks But I've only just started playing the organ properly this year, having been appointed organist in September. I've had a few lessons on pedals so far (the main new thing to learn about when transferring from piano), and am aiming for Grade 6 at the moment.
I've had a look at the Trinity syllabus. I see it's just a recital with optional viva voce. A few of the viola pieces I know already, so it could be useful.

Since it is well known that the jump from grade 8 to diploma level is quite large, surely the ABRSM should have a certificate level exam to plug this gap. It just makes so much sense, surely?

Why?
There are 2 years (normally) between GCSE and A level and a further 3 to 4 years to complete a degree course.
If there was something else between grade 8 and diploma would this not take off the development time needed to get ot diploma level?
All levels are not equally spaced and as you progress it takes longer to get to the next level.
(Funnily enough this is also true in computer games ).

i kind of agree.

i think that all that is needed is another year to develop musically rather than an actual qualification....??? i dunno